The present invention generally relates to a cathode-ray tube for displaying images by the application, upon the phosphor, of the electrons generated by the cathode within a vacuum cell, and more particularly, to an image display element using a cathode-ray tube which has a potential gradient within the vacuum cell.
Conventionally, although a Braun tube is mainly used as a display element for a color television image display, it is impossible to make the conventional Braun tube thinner, because the depth thereof is much greater than the size of the picture face.
As a plate-shaped display apparatus having the same light emitting principle as that of the Braun tube, the present applicant has proposed a novel display apparatus described in U.S. Pat. Nos. 4,451,846 and 4,449,148.
This is a plate type image display apparatus which forwards the electron beams from a plurality of linear thermal cathodes and causes the electron beams, controlled by an electron beam control electrode, to collide against the fluorescent screen so as to display letters, images and so on.
The plate type image display apparatus is constructed as shown in FIG. 1. In FIG. 1, a back electrode 51 is adapted to direct in the front face direction an electron beam 72 emitted from a plurality of linear cathodes shown by reference numerals 52a through 52d. An electron beam forwarding electrode 53 forwards the electrons from the linear thermal cathodes 52a through 52d. Through holes 62 are provided in the electrode 53 to let the electron beams 72 pass through them. A signal electrode 54, which is provided to apply the video signals, is composed of a plurality of control electrodes 64. Each control electrode 64 has through holes 63 therein to let the electron beams 72 pass through it. A first focusing electrode 55 and a second focusing electrode 56 are provided to focus the electron beams 72 in the horizontal and vertical directions.
Through holes 65 and 66 are provided in the electrodes 55 and 56 to let the electron beams 72 pass through them. A horizontal deflection electrode 68 deflects the electron beams in the right and left directions of the picture face, and is composed of one set comb type of electrodes 57a and 57b. The comb type of electrodes 57a and 57b constitute a slot 67 to let the electron beams 72 pass through between the mutual electrodes. A vertical deflection electrode 71 is provided to deflect the electrode beams 72 in the vertical direction of the picture face, and is composed of a set of comb type of electrodes 58a and 58b. The comb type of electrodes 58a and 58b constitute a slot 70 with their mutual electrodes to let the electrode beams 72 pass through them. A face plate (surface glass cell) 60 has a screen 73 composed of a three color phosphor layer of red, green, blue, a black stripe layer provided among them, and a metal back layer provided behind them on the inner face thereof.
A metallic plate 61 forming a back cell and the face plate 60 are joined to form a vacuum cell.
But in such a conventional display apparatus as described hereinabove, the rearward dispersed electrons generated by the electrons applied upon the metal back layer of the face plate 60 in the interior of the display element have a potential gradient, instead of being equipotential like in the conventional Braun tube, and are applied again upon the face plate, thus resulting in the largest factor for lowering the contrast ratio.
The above described factors will be described again hereinafter with reference to the drawings.
FIGS. 2A and 2B are views showing the internal construction of a Braun tube and the present image display element of the above described conventional embodiment.
In the drawings, the portions which are not necessary for illustration are omitted. In the case of the Braun tube, the electron beams 82 transmitted from the electron gun 81 are applied upon the metal back 84 positioned on the face 83. Approximately 80% of the electron beams pass through the metal back 84 and are incident on the fluorescent screen applied upon the face 83 so as to emit the light.
But the approximately 20% remaining electron beams 82 are reflected from the metal back 84 and become rearward dispersed electrons 85 and they are absorbed by a funnel 86 and a shadow mask. This is because the interior of the funnel 86 is equipotential. Although the approximately 20% of the electron beams 82 transmitted from the cathode within the electrode 88 become the rearward dispersed electrons 85 as in the Braun tube, in the case of the present image display element of the conventional embodiment, a high voltage of approximately 10 KV is applied upon the above described metal back 84 with the electrode 88 being provided with approximately 300V. The element has an electrode gradient therein. The rearward dispersed electrons 85 are applied again upon the metal back 84 on the face 83, and the fluorescent screen other than the plate where the electron beams 82 were primarily incident emits light, thus reducing the contrast ratio considerably. It is to be noted that the metal back 84 is composed of an aluminum layer.